I can understand why your friend may think that as white people's hair generally is thinner and more manageable when curly so it is easier to straighten when compared to curly hair of a black person so you do tend to see more black people with curly hair.

However there is no truth in that fact, i'm white and have curly hair and in ireland it is very common, if not more usual to have curly hair than not.

Hope this has helped